Rubinar 500/8 - a low cost flat field apo alternative?

[rmuhlack]

Finally got a chance to try my new Rubinar 500mm f8 Maksutov lens (i believe from Russian producer LZOS) for astroimaging.

This is a quick run taken in the small window this morning between moonset and astronomical twilight. Didn't bother with Pixinsight for this quicky - just calibrated and stacked with DSS and adjusted levels with Darktable.

19 x 4min guided subs @ ISO800 on the NEQ6

larger image here (http://www.astrobin.com/30641/)

Managing fine focus control is a little challenging, so i'm thinking of getting one of these (http://www.teleskop-express.de/shop/product_info.php/info/p3284_Microfocuser-and-focus-lock-fuer-camera-lenses-up-to-D-105mm.html) together with a B mask to assist there. So aside from the fiddly focus issue and some slight field rotation (need to do a better job in future of aligning the lens with the guide scope), i'm pretty happy with how this turned out.

For a lens that cost me less than $200 delivered, i reckon it has a lot of promise as a low-cost flat-field "apo" alternative. Only weighs ~650 grams, so it could be an option for those looking for something light and portable for an Astrotrac or similar...?

[Poita]

The f8 is a bit limiting, but the image looks good!
There is an f5.6 version as well.

Where did you purchase it from?

[rmuhlack]

true, but with a low cost alternative like this there's got to be a compromise somewhere

Mind you, there are plenty of people shooting deep-sky with f6-f8 refractors as their main imaging scopes. ;)

[Wavytone]

Hi Richard, good effort - I used to have the bigger brother of your lens - an MTO 1000mm f/10 and took some astrophotos with it back in the days of film.

[rmuhlack]

Got the lens from eBay (shipped from the UK). At the time there weren't any of the f5.6 versions at a sensible price, and I was concerned about the weight - i believe its about 1.2kg for the f5.6 version.

I got this lens to double as a wildlife telephoto, and wasn't too keen about carrying a 1kg+ lens around with me !!

[Wavytone]

Yes they are massive beasties. The MTO lens has a TWO-ELEMENT achromatic corrector (!) with about 4cm of solid glass up front. I expect yours does, too. Take a close look at the reflections you can see in the front corrector...

BTW is the corrector coated, or uncoated ? Mine was uncoated and after 4 surfaces was losing about 50% of the incoming light through reflections.

[rmuhlack]

Both the front and rear correctors appear to be coated.

[bojan]

Very good purchase, and good price/performance for those russian lenses !

I have all 3 of them (Rubinar 500, Rubinar1000 and MTO-1100A).
The best of the lot (from the image point of view) is MTO, and I am using it as a main scope for imaging (and for spectroscopy in the near future).
See here:
http://www.iceinspace.com.au/forum/showthread.php?t=37039&highlight=MTO

http://www.iceinspace.com.au/forum/showthread.php?t=37509&highlight=MTO

http://www.iceinspace.com.au/forum/showthread.php?t=36606&highlight=MTO

However, I needed to do a slight modification to it's optics, according to the link below (to remove the astigmatism of the main mirror, I relaxed the uneven pressure on glass by adding 3 contact pads on the mirror rim, aligning pads on retaining ring with those and reducing the mirror retaining ring pressure by screwing it on only lightly, and without grab screw at the side. Relaxing the retaining rings of the correction system did not improve the image, due to the glass thickness (as pointed out by Wavytone, there is a lot of thick glass there - perhaps this is the reason for 50% transmission, glass appears to be a bit yellowish ? ). This significantly improved the image quality (VERY visible on individual star images).
Another big improvement was achieved by the way the lens tube is mounted (again, there is a hint on that on Strickling's website (http://www.strickling.net/russentonne.htm (http://www.strickling.net/russentonne.htm)), and here: http://www.harpoint-observatory.com/deutsch/eigenbau/russentonne.pdf).

And, of course, you need to allow the whole system to be in thermal equilibrium - any temperature differences will significantly deteriorate the image quality .

I still need to do the same thing on Rubinar 500 and Rubinar 1000 (a bit tricky because the retaining rings are glued with shellac or something similar, I am not sure how to remove it without damaging the mirror).
Interestingly, corrector plates on those two lenses are much thinner.

Attached image of Tarantula is done last week with MTO-1100A and Canon 60D (stack 12x 45sec ISO3200).

[ZeroID]

I've been looking at these (or similar) on Deal Extreme. I've seen a 500 f6.3 at about $170US and an f8 at $119US. On my A77 2/3 sensor it work work out as 750mm and a heck of a lot cheaper than an APO.

I have the new Tamron 18-200 (24-300) and my old KM 70-300 (105-450) lenses to cover most uses and a cheapish 500mm (750mm) Mak lens would make an interesting addition to play with.

Have to try the Tamron and the KM guided when the clouds naff off.

[bojan]

It wouldn't.
500mm FL is always 500mm FL, no matter what size of sensor you have.
However, your field of view would be 1.6x smaller.

Yes, this solution is much cheaper than APO... but good APO will be better in terms of image quality (sharpness - images of individual stars will be smaller and rounder).
It all depends on what you want from your optical system - those Maksutovs are good enough for many tasks ( light collection capability in general).. but many people from this (and other forums) will tell you they are not good enough for what they do ("pretty pictures"). My MTO is 100% adequate for what I want to do with it, though.

[ZeroID]

Cheers for the clarification Bojan, I'll keep saving for an APO then ...

[OzEclipse]

Miloslav Drückmuller uses one for some of his solar eclipse photography.

He also has used the 500 f5.6 for this remarkable image of comet Holmes.

http://www.zam.fme.vutbr.cz/~druck/Astro/Holmes/2007_11_04-05a/0-info.htm

This type of lens can be difficult to focus accurately, not impossible just harder.

Joe

[mithrandir]

Brent, I have a 500mm f6.3 Samyang CAT and it's a real pain to focus. It works OK when you get it focused, but it's not easy and the bokeh sucks.
However it's much lighter and vastly more compact than my 150-500mm Sigma. On the other hand the Sigma is faster and has A/F. It also has builtin IS which our Sony a77 (and a200) bodies don't need.
Then I found on Sunday that my son bought the same Sigma for his Nikon.

[ZeroID]

Cheers Andrew. Yeah the 'doughnut' bokeh is supposed to be a bugbear for CATs, could be confusing when you are focusing as well methinks. (Is that bokeh or is that defocussed ? :shrug: )
I'm finally getting some decent ( not brilliant ) results with the 80mm Achro on the A77. Even just single hi res jpeg frames are good, much lower noise than I expected at quite high ISOs and I've got PHD guiding under control so I guess the next step is an APO to move up in quality and mag.

$$$$$$ ( :shrug: ) I WAS warned about this before I started down the slippery slope .... :rofl:

[bojan]

Now that I saw Joe's reply I realised I forgot to mention my recent TSE shots from Cairns (http://www.iceinspace.com.au/forum/showthread.php?t=99304&highlight=knob).
They were done with MTO-1100A.

Looking at Miloslav's images, I noticed he was using some other Zenit lenses for his work (Tair 3A, for example, then Rubinar 10/1000...).
I also experimented a lot with Tair-11A (135mm, F2.8) and found that it is exceptionally sharp, with very little (if any) CA. Only a bit of coma at the corners (of APS sensor.. probably significantly worse on FF.. All in all, better that Canon equivalent).

So, there is no real need to spend M$ for good results...

[ZeroID]

I think I'm going to keep my eyes open on the local TradeMe site for older long focal length primes. I've seen them go for only tens of dollars sometimes. A nice Takumar eg. APO's seem to go for a bit more but you never know what will come up.
The A77 seems to be very low noise and it can cope with quite small apertures by bumping ISO up plus I've got PHD working well now.